1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus and more specifically relates to a sheet feeding apparatus and an image forming apparatus which separate and feed sheets by making air blow the sheets.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a printer and a copying machine has a sheet feeding apparatus which feeds sheets loaded on a tray supporting a plurality of sheets one by one. Such a sheet feeding apparatus adopts an air feeding method in which air blows an end portion of a sheet bundle supported on the tray to cause several sheets to be blown up, and in which a sheet is adsorbed by an adsorbing and conveying belt arranged on the upper side to feed the sheets one by one (refer to U.S. Patent Application Publication No. 2005/0206068 A1).
The sheet feeding apparatus adopting this air feeding method makes air blow the end portion on the front end side of the sheet bundle on the tray, causes the sheets to be blown up to loosen the sheets, and makes the adsorbing and conveying belt adsorb the uppermost sheet out of the blown-up sheets by negative pressure. Additionally, the adsorbing and conveying belt adsorbing the sheet is rotated to cause the single sheet to be separated and fed to a downstream. In such a manner, the sheets are separated and fed to an image forming portion one by one. The sheet feeding apparatus in this air feeding method is more durable than a general sheet feeding apparatus in a friction separating method. Thus, the sheet feeding apparatus in this air feeding method is often used in a field of simple book binding (quick printing of booklets or catalogs) with use of the image forming apparatus in xerography enabling Print On Demand (POD).
Recently, there is a demand by users for enhancing productivity (number of images to be formed per unit time) in the image forming apparatus. Especially in the field of the above-mentioned POD (Print On Demand), a large amount of quick printing needs to be performed, and thus the sheet feeding apparatus which has achieved enhancement of productivity is desired. In general, in order to enhance productivity, the number of sheets to be fed from the sheet feeding apparatus per unit time needs to be increased, and to do so, a sheet feeding speed must be raised.
However, to speed up sheet feeding in the sheet feeding apparatus in the air feeding method, the sheets need to be blown up quickly, and a conveying speed of the adsorbing and conveying belt needs to be raised. To cause the sheets to be blown up quickly, an air speed (air quantity) of the blowing air needs to be raised (enlarged). However, when the air speed (air quantity) is raised (enlarged), a large number of sheets will be blown up at a time to disable reliable loosening in a case of thin (low basis weight) sheets. This causes plural sheets to be adsorbed by the adsorbing and conveying belt, which produces an issue of multi feed of the sheets. Also, when the conveying speed of the adsorbing and conveying belt is excessively raised, a thick (high basis weight) sheet will be kept adsorbed firmly by the belt due to an inertial force, which may bring about a jam caused by sheet feeding delay.
The present invention provides a sheet feeding apparatus and an image forming apparatus enabling stable sheet feeding even in a case where productivity of the image forming apparatus is enhanced.